I've finally tracked this down to an old vimspell plugin. Version 1.100 /
released 14-Sep-2005 causes this in conjunction with the Debian vim-gnome
package, version 2:7.2.245-2 (and perhaps others). The solution for me is to
just remove the vimspell plugin: the upstream author recommends that users
of VIM versions 7 and upwards use the built-in spell checker instead anyway.